1. Field of the Invention
The present invention relates to a semiconductor electronic device, in particular to an electronic device which employs a diamond crystal at least for a heat-radiating layer.
The present invention also relates to a process for producing the above electronic device.
2. Related Background Art
The formation of a semiconductor crystal on a heat radiation layer of an insulator is an important technique to achieve higher response speed of electronic elements. Various materials are being investigated for the insulator. Of the materials, diamond is attracting attention because of its excellent characteristics, which are not achieved by other materials, such as larger band gap (5.5 eV), larger carrier mobility (1800 cm.sup.2 /V.multidot.S for electrons, and 1600 cm.sup.2 /V.multidot.S for positive holes), suitability for a heat sink (or heat-releasing material) owing to higher thermal conductivity (2000 W/(m.multidot.K)), excellent insulation properties (not less than 10.sup.16 .OMEGA..multidot.cm), high hardness, high abrasion resistance, and so forth.
Accordingly, synthesis of diamond from a vapor phase is being developed, in particular, by chemical vapor deposition (CVD). Japanese Patent Application Laid-Open No. 2-110968 discloses formation of a silicon semiconductor layer on a diamond insulator.
Hitherto, diamond crystals shown below have been formed on a substrate by a CVD process.
(1) A diamond crystal is formed by homoepitaxial growth on a natural or artificial diamond base or by heteroepitaxial growth on a cubic boron nitride (c-BN) base having a crystal structure close to that of the diamond crystal. Both of thus obtained diamond crystal films are in epitaxial relation to the underlying base material and are a monocrystalline film having an excellent surface smoothness. PA1 (2) A diamond crystal is formed under ordinary synthesis conditions on a silicon substrate, a high melting-point substrate such as molybdenum, tungsten and tantalum, or a quartz substrate. In the synthesis of such a diamond crystal, the substrate is usually subjected preliminarily to scratching treatment with abrasive diamond grains in order to increase nucleation density. The scratching treatment by use of diamond grains was reported by Iijima et al. (Preprint of 4th Diamond Symposium, p. 115 (1991). According to this report, the scratching treatment causes embedding of fine diamond particles on the surface of the substrate, and these particles serve as nuclei in diamond crystal formation. On the fine diamond particles as the nuclei, the diamond crystals deposit in vapor phase synthesis as below depending on the nucleation density: (i) at a low nucleation density, diamond crystals deposit in a grain form or in random orientation, and (ii) at a high nucleation density, diamond crystals deposit in a polycrystal form having a rough surface. It is known that the diamond polycrystalline film is not oriented generally, but can be made oriented in {100} or {110} direction by selecting the synthesis conditions. PA1 (3) A diamond crystal is formed on a monocrystalline copper plate as the substrate. The crystal has the same crystal orientation as that of the substrate according to Japanese Patent Application Laid-Open No. 2-160695. PA1 (1) The monocrystalline film which is formed by heteroepitaxial growth on diamond crystal or on cubic boron nitride is not suitable for practical uses because of extreme expensiveness although it has excellent surface smoothness and high crystallinity. PA1 (2) In the formation of diamond crystal under ordinary synthesis conditions on a silicon substrate, a high melting-point metal substrate, or a quartz substrate, (i) at a low nucleation density, a monocrystal is formed but it is deposited in random orientation directions or (ii) at a high nucleation density, a crystal is obtained usually in a form of a non-oriented polycrystalline film having remarkable surface roughness. If an oriented crystalline film is obtained, the resulting {110}-oriented film has remarkable surface roughness or the resulting {100}-oriented film has inevitably a large thickness (several ten .mu.m or more) for orientation although its surface is parallel to the substrate and has good surface smoothness. PA1 (3) The diamond crystals which are formed on a monocrystalline copper substrate are in a particle shape, and further grow and come contact into each other to become a film with roughness although the film is deposited in epitaxial relation to the substrate.
The diamond crystals formed by the above conventional processes have disadvantages below.
When a semiconductor electronic device is formed on the diamond crystal or the diamond crystal film of the above item (2) or (3), the surface roughness of the diamond may adversely affect the yield of the device formation, or the crystal grain boundary may give an adverse effect of lowering the performance of the device.